Depending upon the design principle, a damping valve which is adjustable by means of magnetic force adopts a particularly soft or a particularly hard damping force characteristic in deenergized state. This effect is produced at least by a spring force against which the magnetic force must act. In case of outage of the magnetic force, e.g., due to cable breakage, the spring force then displaces a valve body into an end position. Assuming that a high damping force also represents high safety reserves for the chassis of a vehicle, then the basic operating principle is advantageous, since the safety device for the adjustable damping valve already exists in practice without further expenditure.
However, it is already known from U.S. Pat. No. 5,413,196 or U.S. Pat. No. 5,085,299 A1 that it is not necessarily the hardest damping force setting that is sought as an emergency operation characteristic, but rather a moderate damping force characteristic. In U.S. Pat. No. 5,413,196, FIG. 2 shows an adjustable damping valve device which comprises an auxiliary valve device and a main stage valve. The damping force of the damping valve device is adjusted through the flow-off of damping medium from a control space of the main stage valve.
The auxiliary valve device comprises an auxiliary control valve and an emergency operation valve. The two valves are arranged in series with respect to the flow path of the damping medium from the control space (FIG. 2). Even with a small energizing current, the emergency operation valve (529a) opens and completely releases the flow-off (26i). Only cross section 26i works in the emergency operation position and therefore determines, e.g., a medium damping force setting.
In the embodiment of FIG. 6 in U.S. Pat. No. 5,413,196, the auxiliary valve device likewise comprises an emergency operation valve, which, however, is hydraulically connected in parallel to the auxiliary valve. This poses the problem that a malfunction of the emergency operation valve also affects the damping force setting of the auxiliary valve.
The construction principle in FIG. 2 and FIG. 6 presents the difficulty that the axially movable valve body of the auxiliary valve and the emergency operation valve body are in direct contact, and the emergency operation valve carries out a displacing movement on the valve body of the auxiliary valve starting from the emergency operation position into the normal operation position.
It is compulsory that the two valve bodies of the auxiliary valve device be able to move very smoothly relative to one another. This results in small annular gaps which, on the one hand, bring about internal leakage which influences the damping force adjustment and, on the other hand, impair the guiding of the valve body relative to the magnetic coil.
These difficulties can be overcome, but the expenditure on fabrication required for this purpose adds to production costs.
US/2010/252766 and DE 10 2009 002 582 A1 disclose a damping valve device having an annular valve body that releases an outlet opening even when energized only slightly. The valve body and the outlet opening form an emergency operation valve. In the event of power outage with closed emergency operation valve, the damping force is determined by a pressure-relief valve which is hydraulically connected to the emergency operation valve in parallel with the latter. In the respective constructions according to FIG. 2, the pressure-relief valve is formed by a small valve cartridge which is arranged radially in an intermediate wall of the damping valve device. However, this radial arrangement of the pressure-relief valve affects both the radial and axial installation space of the damping valve device.
It is an object of the present invention to optimize the installation space requirement of an adjustable damping valve device.